[thejeff]

I was playing around with Jotun Jarls to test this and though I haven't yet got a solo Jarl to miss I did get one to cast a bless that only hit already blessed Woodsmen.

It looks to me like it might be an artifact of the AI determines which spell to cast and where.
It essentially runs multiple tries and picks the one with the best results, usually most damage, possibly most units blessed in this case. Then when it actually casts the spell it rolls all the dice again so the results can be different.
It targets units within range and rather than checking whether that unit can be blessed it just sees if casting bless on that unit results in units getting blessed. With the Dai Oni's wolf or my already blessed Woodsman, I suspect the test case hit a sacred unit, so it went ahead and cast on that target. With different random splash it missed all the unblessed sacreds.

This isn't necessarily a bad thing. If you had sacreds clustered around a non-sacred, targeting the non-sacred might actually get you more blessings.

It doesn't explain my strong memories of Jarls not blessing themselves, but I could be wrong. That may have only happened on defense when there was some PD around.